1. Field of the Invention
The present invention relates to a positioning apparatus for positioning the center of an optical fiber or other small dimensioned cylindrical member, such as capillary tubing, along a predetermined reference axis independently of variations in the outside diameter of the member.
2. Description of the Prior Art
Devices are known for positioning an optical fiber so that the axis of the fiber is positioned with respect to a reference axis. A typical expedient used in such devices is a generally V-shaped groove that is formed in a substrate material and which serves as a cradle to accept the fiber being positioned. Representative of such devices is that shown in U.S. Pat. No. 4,756,591 (Fischer et al.), wherein a V-groove is formed in a silicon substrate and an elastomeric member is biased against the fiber to hold it in the groove. The groove may be stepped to provide a deeper groove segment to hold the jacket of the fiber within the device.
U.S. Pat. No. 4,756,591 (Sheera) discloses a grooved silicon substrate having a pair of intersecting V-grooves therein. A fiber 10 be positioned is disposed in one of the grooves while a shim is disposed in the other of the grooves. The shim may take the form of a tapered or an eccentric fiber, which when respectively slid or rotated under the first fiber serves to lift the same to bring the axis thereof into alignment with a reference axis. A cover may be positioned above the substrate to assist in clamping the first fiber into position.
U.S. Pat. No. 4,802,727 (Stanley) also discloses a positioning arrangement for optical components and waveguides which utilizes a V-grooved structure. U.S. Pat. No. 4,826,272 (Pimpinella et al.) and U.S. Pat. No. 4,830,450 (Connell et al.) discloses arrangements for positioning an optical fiber that utilize members having frustoconical apertures therethrough.
It is believed that single crystalline silicon is the material of choice of the devices above mentioned because of the proclivity of crystalline silicon to be etched along precise crystallographic planes, thus forming precise grooves or structural features by photolithographic microfabrication techniques. Etchants exist that act upon a selected crystallographic plane to a differential degree than upon an adjacent plane, permitting the needed precise control. V-grooves, in particular, can be etched to a controlled width and truncated depth. Under some conditions V-grooves may be etched in a self-limiting operation. The photolithographic microfabrication process is generally described by Brodie and Muray, "The Physics of Microfabrication", Plenum Publishing, New York (1982).
Optical fibers include an inner core having a predetermined index of refraction surrounded by a cladding layer of a lower index. The inner core is the medium in which the optical energy is guided, while the cladding layer defines the index boundary with the core. The outer diameter of the fiber may vary in dimension about a predetermined nominal dimension. It has been seen, for example, that two nominally identical fibers from the same manufacturer may vary in outside diametrical dimension by as much as plus or minus four (4) micrometers. This fiber to fiber variation in outer diameter makes difficult the accurate positioning of the axis of the core of a fiber with respect to a predetermined reference axis using a positioning apparatus having a V-grooved structure.
In view of the foregoing it is believed advantageous to make use of the ability of microfabrication techniques to form accurate structures, channels and/or surfaces in a crystalline material to construct a positioning apparatus that will accurately position the center of the fiber, or of any other elongated generally cylindrical member having small dimensions (such as capillary tubing), with respect to a predetermined reference axis. Moreover, it is believed advantageous to provide a positioning apparatus that consistently aligns the predetermined point on the fiber or other cylindrical member with the reference axis without requiring great technical skill, expensive apparatus, and extensive alignment procedures.